Signal processing in hearing aids is usually implemented by determining a time-varying gain for a signal, and then multiplying the signal within by the gain. This approach gives a linear time-varying system, that is, a filter with a frequency response that changes over time. This system can be very effective for those types of processing, such as dynamic-range compression and noise suppression, where the desired signal processing is a time- and frequency-dependent gain. But because of its linear nature, a time-varying filter cannot be used to implement nonlinear processing such as frequency lowering or phase randomization.
An alternative approach is to use an analysis/synthesis system. For the analysis the incoming signal is usually divided into segments, and each segment is analyzed to determine a set of signal properties. For the synthesis, a new signal is generated using the measured or modified signal properties. An effective analysis/synthesis procedure is sinusoidal modeling known from U.S. Pat. No. 4,885,790, U.S. RE 36,478 and U.S. Pat. No. 4,856,068. In sinusoidal modeling the speech is divided into overlapping segments. The analysis consists of computing a fast Fourier transform (FFT) for each segment, and then determining the frequency, amplitude, and phase of each peak of the FFT. For the synthesis, a set of sinusoids is generated. Each sinusoid is matched to a peak of the FFT; not all peaks are necessarily used. Rules are provided to link the amplitude, phase, and frequency of a peak in one segment to the corresponding peak in the next segment, and the amplitude, phase, and frequency of each sinusoid is interpolated across the output segments to give a smoothly varying signal. The speech is thus reproduced using a limited number of modulated sinusoidal components.
Sinusoidal modeling provides a framework for nonlinear signal modifications. The approach can be used, for example, for digital speech coding as shown in U.S. Pat. No. 5,054,072. The amplitudes and phases of the signal are determined for the speech, digitally encoded, and then transmitted to the receiver where they are used to synthesize sinusoids to produce the output signal.
Sinusoidal modeling is also effective for signal time-scale and frequency modifications as reported in McAulay, R. J., and Quatieri, T. F. (1986), “Speech analysis/synthesis based on a sinusoidal representation”, IEEE Trans. Acoust. Speech and Signal Processing, Vol ASSP-34, pp 744-754. For time-scale modification, the frequencies of the FFT peaks are preserved, but the spacing between successive segments of the output signal can be reduced to speed up the signal or increased to slow it down. For frequency shifting the spacing of the output signal segments is preserved along with the amplitude information for each sinusoid, but the sinusoids are generated at frequencies that have been shifted relative to the original values. Another signal manipulation is to reduce the peak-to-average ratio by dynamically adjusting the phases of the synthesized sinusoids to reduce the signal peak amplitude as shown in U.S. Pat. Nos. 4,885,790 and 5,054,072.
Sinusoidal modeling can also be used for speech enhancement. In Quatieri, T. F, and Danisewicz, R. G. (1990), “An approach to co-channel talker interference suppression using a sinusoidal model for speech”, IEEE Trans Acoust Speech and Signal Processing, Vol 38, pp 56-69 sinusoidal modeling is used to suppress an interfering voice, and Kates (reported in Kates, J. M. (1994), “Speech enhancement based on a sinusoidal model”, J. Speech Hear Res, Vol. 37, pp 449-464) has also used sinusoidal modeling as a basis for noise suppression. In the above mentioned Kates study, the high-intensity sinusoidal components of the signal assumed to be speech were reproduced but low-intensity components assumed to be noise were removed; however, no benefit in improving speech intelligibility was found. Jensen and Hansen (reported in Jensen, J., and Hansen, J. H. L. (2001), “Speech enhancement using a constrained iterative sinusoidal model”, IEEE Trans Speech and Audio Proc, Vol 9, pp 731-740) used sinusoidal modeling to enhance speech degraded by additive broadband noise, and found their approach to be more effective than the comparison schemes such as Wiener filtering.
Sinusoidal modeling has also been applied to hearing loss and hearing aids. Rutledge and Clements (reported in U.S. Pat. No. 5,274,711) used sinusoidal modeling as the processing framework for dynamic-range compression. They reproduced the entire signal bandwidth using sinusoidal modeling, but increased the amplitudes of the synthesized components at those frequencies where hearing loss was observed. A similar approach has been used by others to provide frequency lowering for hearing-impaired listeners by shifting the frequencies of the synthesized sinusoidal components lower relative to those of the original signal. The amount of shift was frequency-dependent, with low frequencies receiving a small amount of shift and higher frequencies receiving an increasingly larger shift.
It is thus an object to provide a computationally simple way of providing stability improvements in a hearing aid.
According to some embodiments, the above-mentioned and other objects are fulfilled by a first aspect pertaining to a hearing aid comprising an input transducer, a high pass filter, a low pass filter, a synthesizing unit, a combiner, a hearing loss processor, and a receiver.
The input transducer is configured for provision of an input signal, such as an electrical input signal.
The high pass filter is configured for providing a high pass filtered part of the input signal. The high pass filter may be connected to the input transducer.
The low pass filter is configured for providing a low pass filtered part of the input signal. The low pass filter may be connected to the input transducer.
The synthesizing unit is configured for generating a synthetic signal. The generation may be based on the high pass filtered part by utilizing a model based on a periodic function. Furthermore, the phase of the synthetic signal may at least in part be randomized. The synthesizing unit may be connected to the output of the high pass filter.
The combiner may be configured for combining the low pass filtered part with the synthetic signal such that a combined signal is provided. The combiner may be connected to the output of the low pass filter and connected to the output of the synthesizing unit.
The hearing loss processor may be configured for processing the combined signal for provision of a processed signal. Alternatively, the hearing loss processor may be configured for providing the processed signal by processing the low pass filtered part and the synthetic signal before combining the respective processed results by means of the combiner. The processing of the hearing loss processor may be in accordance with a hearing loss of a user of the hearing aid.
The receiver is configured for converting an audio output signal into an output sound signal. The audio output signal may be the processed signal or the audio output signal may be derived from the processed signal.
By creating a synthetic signal from the high frequency part of the input signal and combining this synthetic signal with the low pass part of the input signal is achieved that the high frequency part of the input signal is at least in part de-correlated with the output signal of the combiner, thus leading to increased stability of the hearing aid. By dividing the input signal into low- and high-frequency bands with the help of the high and low pass filters, and generating the synthetic signal only at the high frequencies where it is needed, because feedback in hearing aids mostly is a high frequency phenomena significantly reduces the computational burden. The resultant hearing aid thus has the benefits of high stability combined with a greatly reduced computational burden.
According to some embodiments, the periodic function may be a trigonometric function, such as a sinusoid or a linear combination of sinusoids. Hereby is achieved a simple way of modelling speech, because speech signals comprise a high degree of periodicity, and may therefore according to Fourier's theorem be modelled (or approximated) by a sinusoid, or a linear combination of sinusoids. This way a very accurate and yet computationally simple model of particularly speech signals, may be facilitated. It is understood that the term sinusoid may refer to a sine or a cosine.
The high pass and low pass filters may be complimentary, i.e. a pair of low and high pass filters having the same cutoff or crossover frequency.
According to one or more embodiments the frequency of the synthetic signal may be shifted downward in frequency. Hereby is achieved a simple way of further increasing the de-correlation between the input and output signals of the hearing aid.
Alternatively or additionally, the phase of the synthetic signal may at least in part be randomized. This could for example be achieved by replacing the phase of the original (high frequency) signal by a random phase. Hereby an alternative way of providing de-correlation of the input and output signals may be achieved that is computationally simple.
In accordance with some embodiments, the frequency shifting of the synthetic signal may be combined with randomization of the phase. Thus, providing the benefits of de-correlation achieved by frequency shifting and de-correlation provided by phase randomization, simultaneously. Especially, this will lead to higher degree of de-correlation and thereby even further increased stability of the hearing aid.
The randomization of the phase may furthermore be adjustable. This could for example be achieved by blending any desired proportion of the original and random phases. Thus one can introduce the minimal amount of phase randomization needed to produce the desired system (hearing aid) stability, and at the same time giving the highest possible speech quality for the desired degree of stability improvement, while keeping the computational burden as low as possible.
The hearing aid system may according to one or more embodiments comprise a feedback suppression filter placed in a configuration as shown in US 2002/0176584. Hereby is achieved a further increased stability of the hearing aid, thus enabling the use of a higher amplification in said hearing aid before the onset of feedback.
A further aspect of any of the embodiments described herein pertains to a method of de-correlating an input signal and output signal of a hearing aid, the method comprising the following, which may be denoted steps:                dividing the input signal into a high frequency part and a low frequency part,        generating a synthetic signal on the basis of the high frequency part and a model, said model being based on a periodic function, and        combining the synthetic signal with the low frequency part.        
The method may according to one or more embodiments comprise                dividing the high frequency part into a plurality of segments,        windowing and transforming each segment of the plurality of segments into the frequency domain, and        selecting the N highest peaks in each segment,wherein generating the synthetic signal may comprise or may be carried out by replacing each of the selected peaks with the periodic function.        
The segments are according to one or more embodiments overlapping, so that signal feature loss by the windowing may be accounted for.
The step of generating the synthetic signal may further comprise the step of using the frequency, amplitude and phase of each of the N peaks.
The generated synthetic signal may furthermore be shifted downward in frequency by replacing each of the selected peaks with a periodic function having a lower frequency than the frequency of each of said peaks. This could in an alternative embodiment of the method be done for only some of the peaks, i.e. in this alternative embodiment only some frequencies of the selected peaks are replaced with a periodic function having a lower frequency than the frequency of said some peaks.
In accordance with some embodiments, the phase of the synthetic signal is at least in part randomized, by replacing at least some of the phases of some of the selected peaks with a phase randomly or pseudo randomly chosen from a uniform distribution over (0, 2π) radians.
The randomization of the phases may according to one or more embodiments of the method be adjustable. The randomization of the phases may, furthermore or alternatively, be performed in dependence of the stability or stability requirements of the hearing aid.
The periodic function, referred to in any of the steps of the method, may be a trigonometric function, such as a sinusoid or a linear combination of sinusoids.
A particularly advantageous embodiment pertains to a hearing aid comprising:    an input transducer for provision of an input signal, such as an electrical input signal,    a high pass filter configured for providing a high pass filtered part of the input signal,    a low pass filter configured for providing a low pass filtered part of the input signal,    a modelling unit configured for applying sinusoidal modelling to modify the high pass filtered part for generating a modified high frequency signal, wherein the phase of the modified high frequency signal at least in part is randomized,    a combiner for combining the low pass filtered part with the modified high frequency signal for provision of a combined signal,    a hearing loss processor configured for processing the combined signal, the processing being in accordance with a hearing loss of a user of the hearing aid, and    a receiver for converting an audio output signal from the hearing loss processor into an output sound signal.
The hearing loss processor may be configured for processing the audio input signal in accordance with a hearing loss of the user of the hearing aid.
The high pass filter and the low pass filter may be connected to the input transducer.
The modelling unit may be connected to the output of the high pass filter.
The combiner may be connected to the output of the low pass filter and the output of the modelling unit.
In accordance with some embodiments, a hearing aid includes an input transducer for provision of an input signal, a high pass filter configured for providing a high pass filtered part of the input signal, a low pass filter configured for providing a low pass filtered part of the input signal, a synthesizing unit configured for generating a synthetic signal from the high pass filtered part using a model based on a periodic function, wherein a phase of the synthetic signal is at least in part randomized, a combiner configured for combining the low pass filtered part with the synthetic signal for provision of a combined signal, a hearing loss processor configured for processing the combined signal for provision of a processed signal, and a receiver coupled to the hearing loss processor, wherein the receiver is configured for converting an audio output signal into an output sound signal.
In accordance with other embodiments, a method of de-correlating an input signal and output signal of a hearing aid, includes dividing the input signal into a high frequency part and a low frequency part, generating a synthetic signal based on the high frequency part and a model, the model being based on a periodic function, wherein a phase of the synthetic signal is at least in part randomized, and combining the synthetic signal with the low frequency part.
In accordance with other embodiments, a hearing aid includes an input transducer for provision of an input signal, a high pass filter configured for providing a high pass filtered part of the input signal, a low pass filter configured for providing a low pass filtered part of the input signal, a modelling unit configured for applying sinusoidal modelling to modify the high pass filtered part for generating a modified high frequency signal, wherein a phase of the modified high frequency signal is at least in part randomized, a combiner for combining the low pass filtered part with the modified high frequency signal for provision of a combined signal, a hearing loss processor configured for processing the combined signal, and a receiver for converting an audio output signal from the hearing loss processor into an output sound signal.
Other and further aspects and features will be evident from reading the following detailed description of the embodiments.
While several embodiments of several aspects are described herein, it is to be understood that any feature from one or more embodiments of one of the aspects may be comprised in one or more embodiments of one or several of the other aspects, and when it in the present patent specification is referred to “an embodiment” or “one or more embodiments” it is understood that it can be one or more embodiments according to any one of the aspects.